1. Field of the Invention
The exhaust gas generated by a typical internal combustion engine, as may be found in motor vehicles, includes a variety of constituent gases, including hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) and oxygen (O2). The respective rates at which an engine generates these constituent gases are typically dependent upon a variety of factors, including such operating parameters as air-fuel ratio (xcex), engine speed and load, engine temperature, ambient humidity, ignition timing (xe2x80x9csparkxe2x80x9d), and percentage exhaust gas recirculation (xe2x80x9cEGRxe2x80x9d). The prior art often maps values for instantaneous engine-generated or xe2x80x9cfeedgasxe2x80x9d constituents, such as NOx, based, for example, on detected values for instantaneous engine speed and engine load.
2. Background Art
The exhaust gas generated by a typical internal combustion engine, as may be found in motor vehicles, includes a variety of constituent gases, including hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) and oxygen (O2). The respective rates at which an engine generates these constituent gases are typically dependent upon a variety of factors, including such operating parameters as air-fuel ratio (8), engine speed and load, engine temperature, ambient humidity, ignition timing (xe2x80x9csparkxe2x80x9d), and percentage exhaust gas recirculation (xe2x80x9cEGRxe2x80x9d). The prior art often maps values for instantaneous engine-generated or xe2x80x9cfeedgasxe2x80x9d constituents, such as NOx, based, for example, on detected values for instantaneous engine speed and engine load.
To limit the amount of engine-generated constituent gases, such as HC, CO and NOx, that are exhausted through the vehicle""s tailpipe to the atmosphere as xe2x80x9cemissions,xe2x80x9d motor vehicles typically include an exhaust purification system having an upstream and a downstream three-way catalyst. The downstream three-way catalyst is often referred to as a NOx xe2x80x9ctrapxe2x80x9d. Both the upstream and downstream catalyst store NOx when the exhaust gases are xe2x80x9cleanxe2x80x9d of stoichiometry and release previously stored NOx for reduction to harmless gases when the exhaust gases are xe2x80x9crichxe2x80x9d of stoichiometry. Typically, such traps include ceria, which characteristically operates to store a quantity of available oxygen during the initial portion of lean engine operation.
Under one prior art approach, the duration of any given lean operating excursion (or its functional equivalent, the frequency or timing of each purge event) is controlled based upon an estimate of how much Nx has accumulated in the trap since the excursion began. For example, in U.S. Pat. No. 5,473,887 and U.S. Pat. No. 5,437,153, a controller seeks to estimate the amount of NOx stored in the trap by accumulating estimates for feedgas NOx which are themselves obtained from a lookup table based on engine speed, or on engine speed and load (the latter perhaps itself inferred, e.g., from intake manifold pressure). The controller discontinues the lean operating excursion when the total feedgas NOx measure exceeds a predetermined threshold representing the trap""s nominal NOx-storage capacity. In this manner, the prior art seeks to discontinue lean operation, with its attendant increase in engine-generated NOx, before the trap is fully saturated with NOx, because engine-generated NOx would thereafter pass through the trap and effect an increase in tailpipe NOx emissions.
However, the disclosed NOx-estimating means fails to account for any instantaneous reduction in trap efficiency, i.e., the trap""s ability to store an additional amount of feedgas NOx. The disclosed NOx-estimating means further fails to account for the trap""s initial storage of oxygen, which likewise reduces the trap""s overall NOx-storing capacity.
The prior art has also recognized that the trap""s actual or maximum NOx-storage capacity is a function of many variables, including trap temperature, trap history, sulfation level, and thermal damage, i.e., the extent of damage to the trap""s NOx-absorbing materials due to excessive heat. See, e.g., U.S. Pat. No. 5,437,153, which further teaches that, as the trap approaches its maximum capacity, the incremental rate at which the trap absorbs NOx may begin to fall. Accordingly, U.S. Pat. No. 5,437,153 teaches use of a nominal NOx capacity which is significantly less than the actual NOx capacity of the trap, to thereby theoretically provide the trap with a perfect instantaneous NOx-absorbing efficiency, i.e., the trap absorbs all engine-generated NOx, as long as stored NOx remains below the nominal capacity. A purge event is scheduled to rejuvenate the trap whenever accumulated estimates of engine-generated NOx reach the nominal trap capacity. Unfortunately, however, the use of such a fixed nominal NOx capacity necessarily requires a larger trap, because this prior art approach relies upon a partial, e.g., fifty-percent NOx fill in order to ensure absorption of engine-generated NOx.
Unfortunately, empirical evidence suggests that the instantaneous storage efficiency of the trap, i.e., the trap""s instantaneous ability to absorb all of the NOx being generated by the engine, rarely approaches 100 percent. Indeed, as the trap begins to fill, the instantaneous storage efficiency of the trap appears to decline significantly, with an attendant increase in the amount of NOx being exhausted to the atmosphere through the vehicle""s tailpipe. While increasing the frequency of the purge events may serve to maintain relatively higher trap storage efficiencies, the fuel penalty associated with the purge event""s enriched air-fuel mixture and, particularly, the fuel penalty associated with an initial release of oxygen previously stored in the three-way catalyst during lean engine operation, would rapidly negate the fuel savings associated with lean engine operation.
Moreover, under certain engine operating conditions, for example, under high engine speed and high engine load, the NOx generation rate and correlative exhaust flow rate through the trap are both so high that the trap does not have an opportunity to store all of the NOx in the exhaust, even assuming a 100 percent trap storage efficiency. As a result, such operating conditions are themselves typically characterized by a significant increase in tailpipe NOx emissions, notwithstanding the use of the NOx trap.
When the engine is operated using a fuel containing sulfur, SOx accumulates in the trap to cause a decrease in both the trap""s absolute NOx capacity and the trap""s instantaneous efficiency. When such trap sulfation exceeds a critical level, the accumulated SOx must be xe2x80x9cburned offxe2x80x9d or released during a desulfation event, during which trap temperatures are raised above perhaps about 650xc2x0 C. in the presence of excess HC and CO. By way of example only, U.S. Pat. No. 5,746,049 teaches a trap desulfation method which includes raising the trap temperature to at least 650xc2x0 C. by introducing a source of secondary air into the exhaust upstream of the NOx trap when operating the engine with an enriched air-fuel mixture and relying on the resulting exothermic reaction to raise the trap temperature to the desired level to purge the trap of stored SOx.
It is an object of the invention to provide a method and apparatus for controlling the filling and purging of a NOx trap which can more accurately regulate overall tailpipe NOx emissions than prior art methods and apparatus.
In accordance with the invention, a method is provided for controlling the operation of a lean-burn internal combustion engine, the exhaust gas from which is directed through an exhaust purification system including a lean NOx trap. Under the invention, during lean engine operation, the method includes determining a value representing an incremental amount, in grams per second, of feedgas NOx generated by the engine as a function of current values for engine speed, engine load or torque, and the lean operating condition""s air-fuel ratio. The method also includes determining a value representing the incremental amount of NOx being instantaneously stored in the trap, preferably, as a function of trap temperature, an amount of NOx previously stored in the trap, an amount of sulfur which has accumulated within the trap, and a value representing trap aging (the latter being caused by a permanent thermal aging of the trap or the diffusion of sulfur into the core of the trap material which cannot be purged).
The method further includes calculating a value representing instantaneous tailpipe NOx emissions based on the difference between the feedgas NOx value and the incremental NOx-storage value; comparing the instantaneous tailpipe NOx emissions value to a predetermined threshold value; and discontinuing the lean engine operating condition when the instantaneous tailpipe NOx emissions value exceeds the predetermined threshold level, either instantaneously or as averaged over the course of a trap purge-fill cycle, whose duration is determined by a timer which is nominally reset to zero upon commencement of an immediately prior rich engine operating condition.
In accordance with another feature of the invention, in a preferred embodiment, the method further includes generating a value representative of the cumulative number of miles that the vehicle has traveled during a given trap purge-fill cycle; and determining a value representing average tailpipe NOx emissions in grams per mile using the instantaneous tailpipe NOx emissions value and the accumulated mileage value.
In a preferred embodiment, the method further includes determining a need for releasing previously stored NOx from the trap; and deselecting the trap-filling lean engine operation in response to the determined need. More specifically, under the invention, determining the need for releasing previously stored NOx includes calculating a value representing the cumulative amount of NOx stored in the trap during a given lean operation condition based on the incremental NOx-storage value; determining a value representing an instantaneous NOx-storage capacity for the trap; and comparing the cumulative NOx-storage value to the instantaneous NOx capacity value. In a preferred embodiment, the step of determining the instantaneous NOx capacity value includes estimating an amount of sulfur which has accumulated within the trap.